A method of manufacturing a resonator within a semiconductor device is described in US patent application US 2002/0145489. Such a resonator provides an integrated oscillator with a very high Q factor that is needed to generate a stable frequency for the systems comprising a semiconductor device such as RF transceivers. Such a resonator is usually called Micro Electromechanical Device or MEM. Such a MEM is intended to replace discrete Quartz oscillators, which are costly devices and which cannot be integrated within a semiconductor device. The semiconductor is based on an SOI (Silicon on Insulator) wafer comprising a substrate. An SOI wafer comprises an insulating layer generally made of oxide. The method of manufacturing such a resonator comprises the steps of:                defining a first layer born on the substrate,        defining a second layer born on the substrate,        etching the second layer to define an element capable of resonating in a vibrational mode, the element being approximated to a trench.        
The oxide layer renders it possible to stop the etching adequately and to avoid a coupling with the substrate, that is to say a leakage current. As is shown in FIG. 33 of said prior art, the element is attached to the substrate at its bottom.
A first drawback of such a method is that it needs a specific complicated processing step after the realization of the resonator to make it hermetic. Indeed, usually, at least one protective oxide layer and at least one metallization layer covers the whole substrate. In the prior art described, there is a need to define additional cap layers in order to prevent the element being cast into the metallization and oxide layers.
A second drawback is that the etching cannot be deep because it is stopped by the oxide insulating layer, such a layer being not deeply integrated inside the substrate of the wafer, from 0.5 to 2 microns, which is a limitation in this SOI technology.
Finally, it is a costly solution because it uses SOI wafers, which are known to be more expensive than conventional semiconductor wafers.